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Abstract:

A method of managing fault recovery in a trunk-branched OADM network may
include determining that an optical power level over data channels of a
first communications link between a first and a second terminal of the
branched optical network exceeds an optical power limit. The method may
further include increasing optical power sent over spare channels of the
first communications link to a first level at which the optical power
level over the data channels decreases to a second level below the
optical power limit.

Claims:

1. A method of managing fault recovery in a branched optical network,
comprising: determining that an optical power level over data channels of
a first communications link between a first and a second terminal of the
branched optical network exceeds an optical power limit; and increasing
optical power sent over spare channels of the first communications link
to a first level at which the optical power level over the data channels
decreases to a second level below the optical power limit.

2. The method of claim 1, wherein the determining comprises measuring a Q
factor for one or more data channels; and determining that the Q factor
on the one or more measured data channels is below a forward error
correction threshold.

3. The method of claim 1, wherein the spare channels comprise guard band
channels occupied by a discrete tone initial loading equipment (DT-ILE)
apparatus associated with a first terminal of the branched optical
network.

4. The method of claim 1, wherein amplifiers within the branched optical
network are configured to maintain optical power over the first
communications link within a first range.

5. The method of claim 4, wherein the optical power over data channels is
set in response to a loss of a second communications link of the branched
optical network. (note: naturally, 5 is resulted of item 4)

6. The method of claim 5, wherein the first communications link is in a
first branch of an OADM network and the second communications link is in
a second branch of the OADM network.

7. The method of claim 1, wherein the determining takes place in the
second terminal, the method further comprising, sending a signal to the
first station to adjust optical power of the spare channels to the first
level.

8. The method of claim 1, wherein the determining and the increasing are
done in an iterative fashion.

9. A system for fault recovery in a branched optical network, comprising:
a discrete tone initial loading equipment (DT-ILE) apparatus configured
to supply power to a set of channels over a first communications link of
the branched optical network, and a control system configured to detect
an optical power level over data channels of the first communications
link, And a control system that detecting the Q degradation over the data
channels of the first communications link wherein the control system is
configured to alert the DT-ILE apparatus to adjust its optical power
output upwardly when the degradation of the data channel performance is
occurred.

10. The system of claim 9, wherein the DT-ILE apparatus and control
system are located at a common terminal of the branched optical network.

11. The system of claim 9, wherein the system is configured to monitor a
Q-factor of the data channels in the first communications link.

12. The system of claim 11, wherein the control system is configured to
alert the DT-ILE apparatus to increase its optical power output when the
monitored Q-factor is below a forward error correction threshold.

13. The system of claim 12, wherein the monitored Q factor comprises a
Q-factor for a predetermined set of data channels.

14. The system of claim 9, wherein the DT-ILE is configured to adjust its
output power upwardly over the set of channels until the optical power
level over the data channels no longer exceeds the first threshold.

15. The system of claim 9, wherein the set of channels are guard band
channels that surround the data channels.

16. The system of claim 9, wherein the control system is configured to
control the adjusting of optical power output of the DT-ILE in an
iterative fashion.

17. The system of claim 9, wherein the branched optical network is an
OADM subsea communications network.

18. An article comprising a computer-readable storage medium containing
instructions that if executed by a processor enable a system to:
determine that an optical power level over data channels of a first
communications link between a first and a second terminal of a branched
optical network exceeds an optical power limit; and increase optical
power sent over spare channels of the first communications link to a
first level at which the optical power level over the data channels
decreases to a second level below the optical power limit.

19. The article of claim 18, comprising instructions that if executed by
a processor enable the system to: measure a Q factor for one or more data
channels; and determine that the Q factor on the one or more measured
data channels is below a forward error correction threshold.

20. The article of claim 18, comprising instructions that if executed by
a processor enable the system to perform the determining and increasing
in an iterative fashion.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] Embodiments of the present disclosure relate to the field of
optical communication systems. More particularly, the present disclosure
relates to a method and system for fault recovery in trunk and branch
optical add/drop multiplexing (OADM) networks.

[0003] 2. Discussion of Related Art

[0004] Undersea fiber optic communication systems may include a main trunk
path extending between land-based cable stations and one or more branch
segments connected thereto. The main trunk is defined by an undersea
cable having a plurality of optical fibers therein and one or more
repeaters or optical amplifiers disposed along the trunk path used to
amplify optical signals transmitted between the cable stations. Each
cable station includes terminal equipment used to transmit and receive
these optical signals along the main trunk path. The one or more branch
segments are coupled to the trunk through a branching unit (BU) at one
end and to a branch segment cable station at another end. These systems
are referred to as trunk and branch networks. Trunk cable stations may be
used to carry information signals through the backbone of the network
while the branch segments may be used to transmit or receive traffic
between the trunk paths to the branch cable stations. The optical signals
transmitted between the trunk and branch cable stations are typically
dense wavelength-division multiplexed (DWDM) signals in which a plurality
of optical channels, each at a respective wavelength, are multiplexed
together.

[0005] Historically long haul undersea trunk-and-branch networks have been
used to provide connectivity between cable stations by using dedicated
fiber pairs. A more recent architecture, employs optical add/drop
multiplexers (OADMs) to provide a more flexible distribution of
transmission connectivity in comparison to technologies using fiber
pairs. In general, an OADM node is used to add and/or drop channels
within a DWDM optical signal between the trunk and branch segments. The
advantages of utilizing OADMs, in part, stems from the ability to share
capacity of dedicated fiber pairs among multiple network branches.

[0006] In typical optical trunk and branch network configurations, not all
the system bandwidth is utilized at initial deployment. Consequently, the
initially deployed data channels may experience higher optical power
which may cause system performance degradation. In these cases, initial
loading equipment (ILE) is employed to transmit non-payload carrying
signals within the system bandwidth or transmission spectrum between
cable stations. In other words, the ILE may be used to fill up un-used
capacity of the network before most or all system bandwidth are deployed
as payload channels.

[0007] ILE may transmit and receive discrete tones which are referred to
as discrete tone initial loading equipment (DT-ILE). Subsequently, as
each payload data channel is being added into the network, the ILE is
replaced by optical terminal equipment which transmit/receive payload
channels within the network. Thus, depending on the amount of system
bandwidth used for data transmission, the ILE may consume more or less
portions of repeater power in the system until all system bandwidth are
utilized by data traffic in the network. In the case of DT-ILE, this
power consumption may take place over particular frequencies or
wavelengths that may border on the high and/or low range of channel
frequencies used for payload transmission. As a result, the insertion of
the ILE signals may also serve the purpose of power management in the
optical network to ensure the installed data channels are at a preferred
power level.

[0008] In OADM trunk and branch networks, optical power management remains
a challenge, especially when there a cable fault occurs. Cable faults
that interrupt traffic, such as cable cuts, can cause transmission loss
between cable stations. This incidence can lead to severe optical power
changes with remaining optical channels in the network. FIG. 1
illustrates a conventional and simplified OADM trunk and branch network
10 including trunk cable stations or terminals 12 and 14 connected via
trunk path 16. Branching units 18 and 20 couple branch cable stations or
branch terminals 30 and 32 to trunk path 16 through respective branch
segments 34 and 36. Each of the branching units 18 and 20 include OADM
nodes used to add/drop channels propagating between trunk path 16 to
branch segments 34 and 36. Trunk path 16 is defined by an optical cable
having a plurality of optical fiber pairs, optical amplifiers 16a,
16a1, 16b, 16b1, 16c, 16c1 disposed along the optical
cable as well as other optical/electrical equipment used to transmit
optical signals along the trunk path 16 from between terminals 12 and 14.
Typically, the optical signals or "through traffic" travel along trunk 16
between terminals 12 and 14, whereas signals destined for branch
terminals 30 and 32 are added/dropped from the trunk 16 using OADM nodes
in branching units 18 and 20 respectively. For each fiber pair along
trunk 16 there are two corresponding fiber pairs within each branch
segment 34, 36 in order to provide transmission capacity in both
directions to/from branching unit 18 and branch terminal 30 as well as in
both directions to/from branching unit 20 and branch terminal 32 thereby
supporting connectivity between all terminals 12, 14, 30 and 32.

[0009] If system 10 is fully loaded and a cable cut 40 occurs along branch
34, the cut may result in an optical power surge associated with the
channels between terminals 12 and 14 in order to maintain the level of
optical power over the system within a preset range. FIG. 2 depicts
possible optical power spectra that may be detected at terminal 12 during
system operation before a cut (50) and after a cut (52). In this example,
data channels (represented by portion 54) are allocated for the traffic
between station 30 and 14. Data channels (represented by portion 56) are
allocated for the traffic between station 12 and 14. The signal power
level 50 before a cut 40 may correspond to a level in which optical
signals are properly transmitted along trunk path 16 without error or
with an error rate within acceptable limits. When the cut 40 occurs, data
channels 54 will be out of service because of the discontinuity of the
fiber path for data channels 54. Meanwhile, data channels 56 remain
propagating in the trunk path 16 with an increased power level. However,
if after cut 40 occurs, the optical signal power level 52 for data
channels 56 exceed a level at which optical signals can be properly
transmitted between terminals 12 and 14, disruption of payload traffic in
data channels 56 in system 10 may result in trunk path 16. In view of the
above it will be apparent that a need exists to remedy undersea OADM
networks when a cable fault occurs, referred as OADM fault recovery.

SUMMARY OF THE INVENTION

[0010] Embodiments of the present disclosure are directed to systems and
methods for management of traffic during fault recovery in optical OADM
networks. In one embodiment, a method of managing fault recovery in a
branched optical network may include determining that an optical power
level over data channels of a first communications link between a first
and a second terminal of the branched optical network exceeds an optical
power limit. The method may further include increasing optical power sent
over ILE channels of the first communications link to a first level at
which the optical power level over the data channels decreases to a
second level below the optical power limit.

[0011] In another embodiment, a system for fault recovery in a branched
optical network includes a discrete tone initial loading equipment
(DT-ILE) apparatus configured to supply power to a set of channels over a
first communications link of the branched optical network. The system
further includes a control system configured to detect an optical power
level over data channels of the first communications link, wherein the
control system is configured to alert the DT-ILE apparatus to adjust its
optical power output upwardly when the optical power level over the data
channels is determined to exceed a first threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic depiction of a known branched optical
network.

[0013] FIG. 2 depicts an optical power spectrum over a communications link
of the branched optical network of FIG. 1 before and after a cut in
another communications link.

[0014]FIG. 3 presents a schematic depiction of a branched optical network
consistent with an embodiment of the disclosure.

[0015] FIG. 4a illustrates an exemplary optical power distribution showing
the spectral position and power over ILE channels in a DT-ILE system, as
well as data channels during initial loading conditions of a
communications link.

[0016] FIG. 4b illustrates the spectral position and optical power of
signals generated by the DT-ILE and data channels after upgrade of the
communications link of FIG. 4a.

[0017] FIG. 4c illustrates the instantaneous distribution of optical power
across data channels and DT-ILE channels of the communications link of
FIG. 4b that may occur after a cut in a line of an OADM system.

[0018] FIG. 4d depicts distribution of optical power across channels of
the communications link of FIG. 4c after adjustment consistent with
embodiments of the present disclosure.

DESCRIPTION OF EMBODIMENTS

[0019] The present invention will now be described more fully hereinafter
with reference to the accompanying drawings, in which preferred
embodiments of the invention are shown. This invention, however, may be
embodied in many different forms and should not be construed as limited
to the embodiments set forth herein. Rather, these embodiments are
provided so that this disclosure will be thorough and complete, and will
fully convey the scope of the invention to those skilled in the art. In
the drawings, like numbers refer to like elements throughout.

[0020] To solve the deficiencies associated with the methods noted above,
novel and inventive techniques for managing optical communication systems
and in particular, fault recovery in a branched OADM optical network due
to one or more cable cuts is disclosed.

[0021] Referring to the drawings, FIG. 3 illustrates an embodiment of an
OADM system 100. The OADM system 100 may be an optical communication
system, such as a subsea communications system. As depicted, OADM system
100 includes trunk terminals (cable stations) 102, 104 that are coupled
to opposite ends of a trunk path 106. The term "coupled" as used herein
refers to any connection, coupling, link or the like by which signals
carried by one system element are imparted to the "coupled" element. Such
"coupled" devices are not necessarily directly connected to one another
and may be separated by intermediate components or devices that may
manipulate or modify such signals. The trunk path 106 may include optical
fiber in a plurality of optical cable segments, e.g. cable segments 108,
110, and 112, for carrying optical signals. Each cable segment may
include one or more sections of optical fiber cable including optical
fiber pairs for carrying bi-directional signals, as well as one or more
amplifiers. In the example shown, cable segment 108 includes amplifiers
120,122, cable segment 110 includes amplifiers 124,126, and cable segment
112 includes amplifiers 128,130. It will be readily appreciated by those
of ordinary skill that the trunk path 106 may include additional
amplifiers depending on its length.

[0022] In various embodiments one or more branching units such as
branching units 114,116 may be coupled to the trunk path 112 between the
trunk terminals 102 and 104, and may include optical add/drop multiplexer
(OADM) apparatus (not separately shown). Accordingly, units 114 and 116
may also be referred to herein as "OADM node 114" or "OADM node 116".
Each OADM node 114,116 may be further coupled to a branch terminal, e.g.,
respective branch terminals 132,134, through a respective branch path
140,142.

[0023] In operation, the branching units 114,116 may direct optical
signals to and from the respective branch paths 140,142 and branch
terminals 132,134 by dropping and adding allocated channel wavelengths.
The branch paths 140,142 may also include optical fiber in optical cable
segments to provide a transmission path for bi-directional communication
of optical signals. The system 100 may therefore provide bi-directional
communication of optical signals between any of the terminals 102, 104,
132, and 134. For ease of explanation, the description herein may refer
to transmission from one terminal to another. It is to be understood,
however, that the system 100 may be configured for bi-directional or
uni-directional communication between any number of the terminals 102,
104, 132, and 134. Similar to the trunk path 106, the branch paths
140,142 may further include one or more amplifiers, as discussed below.
In various embodiments, the trunk path and branch paths of an OADM system
may each contain one or more communications links. The term
"communications link" as used herein may refer to a unidirectional or
bidirectional optical link over one or more optical fibers, that lie
anywhere within the branched optical network. The operation of OADM nodes
114 and 116 may be in accordance with known OADM apparatuses. OADM node
114, for example, may add/drop data channels for transmission, for
example, along trunk path 106.

[0024] As illustrated in FIG. 3, branch path 140 may include amplifiers
152, 154, 156, 158, while branch path 142 includes amplifiers 160, 162,
164, 166. System 100 also includes one or more ILE units 170,172 which
may be deployed at respective trunk stations 102,104 as shown in FIG. 3.
In some embodiments, one or more ILE is configured to continue operation
even when one or more communications links of a path, such as trunk path
106, are fully loaded with data traffic. In various embodiments, a
discrete tone ILE unit (DT-ILE), which employs lasers to provide discrete
laser tones at desired frequencies, may be located in one or more
stations 102, 104, 132, and 134. Except as otherwise noted herein, the
DT-ILE may have conventional components and structure, such as laser(s)
that may produce one or more tones at discrete wavelengths (frequencies)
for which the optical power at each wavelength can be adjusted. The
DT-ILE tones may be transmitted over an optical link together with
optical data signals, as described further below.

[0025] In the embodiment depicted in FIG. 3, a DT-ILE unit 170,172 is
deployed within each of respective terminals 102,104. System 100 further
includes ILE control systems 174, 176, which may be deployed in
respective trunk terminals 102 and 104, as illustrated. An ILE control
system, such as ILE control system 174, may dynamically adjust operation
of an associated ILE, such as ILE 170, in response to changes, such as
fault occurrences, in OADM system 100. In some embodiments, a single
control system may control more than one ILE.

[0026] In operation, one or more ILE units may perform multiple functions
to aid in optical communications across system 100. During initial stages
of deployment, the ILE may function to load unused optical spectrum with
signals that have power density to produce a desired total optical power
over a communication link in system 100. FIG. 4a illustrates an exemplary
optical power distribution 200 over a communications link, which includes
a DT-ILE optical power distribution 202 in a DT-ILE system, as well as a
data channel optical power distribution 204, that may be representative
of during initial loading conditions. As illustrated, there are large
portions of empty spectrum that are not used for data communication. In
this case, the optical power generated by the DT-ILE is relatively strong
to compensate for the lack of occupied spectrum by data channels, which
produce a required optical power distribution 204. During upgrade of a
system, the spectral position occupied by signals generated by the DT-ILE
may shift to compensate for new data channels that may occupy the
frequency formerly used by the DT-ILE, as depicted in optical power
distribution 210 of FIG. 4b. The data channels occupy a fuller range of
spectrum in FIG. 4b and output a larger optical power distribution 214,
which requires that the DT-ILE channels be placed at other frequencies,
as illustrated. For example, the DT-ILE channels may occupy other unused
spectrum or guard band regions. In addition, the optical power
distribution 212 from the DT-ILE may be reduced from that employed during
the initial loading conditions illustrated in FIG. 4a, so that the total
optical power across the spectrum used for an OADM system falls within a
designed range. Thus, when the system channels are fully or nearly fully
loaded with data traffic, the DT-ILE may be tuned down to a minimal power
and may occupy any portion of the data channel spectrum or guard band
region of the spectrum.

[0027] After a communication link is fully loaded, in various embodiments,
the DT-ILE equipment is configured to remain active, even if the optical
power output by the DT-ILE is maintained at a minimum level to limit its
interaction with data channels. In these circumstances, the DT-ILE, such
as ILE 170,172, may act as a system for fault recovery in an OADM
network.

[0028] As noted, under fully loaded conditions, an ILE may remain deployed
but may be tuned down to a minimal power and may have its output channels
placed in a preferable position within the data channels or guard band
region of the spectrum. However, the OADM system may be configured to
dynamically adjust the optical power output of an ILE to respond to
changes in data channels, so that the optical power along given trunk
and/or branch paths remains within an acceptable range. For example,
referring again to FIG. 3, if a cable cut 150 occurs along line 140a
between stations 132 and 114, the loss of optical signals along line 140a
caused by the cable cut 150 may result in an optical power surge along
other paths of system 100, for example, between terminals 102 and 104.

[0029] FIG. 4c illustrates the instantaneous optical power distribution
220 that may occur after a cut in a line of an OADM system such as cut
150. For example, this spectrum may be detected at terminal 102. An
optical power increase, as reflected in data channel optical power
distribution 224, may result automatically from conventional OADM network
equipment that is configured to maintain the total optical power in a
network within a proper range. For example, amplifiers within the OADM
network 100 may be configured to maintain optical power over the trunk
path 106 within a desired range. Thus, a cut in data cable in one portion
of the OADM network may instantaneously reduce overall optical data
channels to the extent that the optical power in remaining data channels
of undamaged portions of the network is automatically increased to offset
the power loss in the damaged portion. However, optical power output in
an undamaged link, represented by the data channel optical power
distribution 224, may be above a range in which data can be properly
transmitted and received, resulting in a loss or degradation of
communication over the "undamaged" link. For example, referring again to
FIG. 3, the cut 150 over line 140a may result in degrading or
extinguishing communications between trunk station 102 and other stations
in the OADM network 100 if the resultant optical power distribution 224
is too high.

[0030] In order to address this situation, in embodiments of the present
disclosure, a DT-ILE may adjust its operation in response to a detected
traffic loss due to optical power increase over data channels of a
communications link. In various embodiments the control system 174 is
configured to send a signal to DT-ILE 170 to adjust its power upwardly in
response to a detected increase in optical power over designated
frequencies of data channels. Thus, when a condition corresponding to the
data channel optical power distribution 224 is detected, control system
174 may determine that the optical power level is too high for proper
communications. In order to reduce the data channel optical power
distribution 224 to the acceptable level, the control system 174 is
configured to send a signal to a DT-ILE of system 100 to adjust the
DT-ILE optical power output in DT-ILE channels upwardly. The DT-ILE then
may increase power for predetermined frequencies to a higher value,
resulting in an adjusted optical power distribution 232 over the DT-ILE
channels, as shown in FIG. 4d.

[0031] Once the optical power distribution 232 is output from a DT-ILE,
the OADM system may automatically adjust the optical power of data
channels downwardly so that the total optical power over a given
communications link remains within a predetermined range. The resulting
data channel optical power distribution 234 may then fall within an
acceptable range for communications. It is to be noted, however, that the
optical power distribution 234 in FIG. 4d need not be the same as that of
optical power distribution 214 in FIG. 4b, as long as the optical power
output is within an acceptable range for data to be effectively
transmitted.

[0032] Thus, in accordance with embodiments of the disclosure, a "tuned
up" DT-ILE may output a relatively higher optical power over a
communication link, such as a fully loaded trunk path, during a period of
fault recovery in an OADM system. Once fault recovery is complete, such
as after repairing a cable cut, the power output of the DT-ILE may be
tuned down to a "warm" stage again in which optical power output is
minimal, according to known operation of a DT-ILE in an OADM system.

[0033] The control systems, such as control system 174, may be at least
partially implemented as a program of instructions on a non-transitory
computer readable storage medium capable of being read by a machine
capable of executing the instructions. Thus, the control system 174 may
execute a program that has as an input, the optical power level of data
channels of a communication link in question. The program may be
configured to determine one or more actions based upon the optical power
level, and to output the appropriate control signals. In various
embodiments, the control system may be embodied in one or more computer
chips that may be included within an ILE, such as DT-ILE 170,172, or may
be included at any convenient location of a branched optical network.

[0034] In various embodiments, the determination that optical power is
excessive on a given communication link may be made by monitoring
channels performance of the communication link. In some embodiments, the
Q-factor of a predetermined set of data channels may be monitored to
determine when the optical power is excessive. In some embodiments, if
the Q-factor is below a forward error correction (FEC) threshold, a
determination may be made that the optical power is too high, which may
trigger the control system 174 to send a signal to the DT-ILE to increase
output power of the DT-ILE. In some embodiments, the increase in power of
the DT-ILE may be done in an iterative fashion. For example, one or more
subsequent scans may be performed to measure whether the Q-factor of all
or selected data channels are above an FEC threshold, and, if not, a
signal is sent to increase output power of the DT-ILE.

[0035] In one specific example, if the optical power of data channels is
initially determined to be too high, the control system 174 may send an
initial instruction to DT-ILE 170 to adjust its optical power output by a
predetermined amount. Once the DT-ILE optical power output increases by
the predetermined amount, other apparatus within the OADM system (such as
amplifiers) may automatically adjust the optical power of data channels
of the communications link downwardly to a new level based upon the
adjusted optical power output of the DT-ILE. If the control system 174
determines that the new level is still too high, the control system may
send a second alert to the DT-ILE 170 to adjust its optical power output
to a still higher level, and so forth, until the optical power of the
data channels is deemed to fall within an acceptable range. As noted
above, this determination may be made by measuring a Q-factor, among
other ways.

[0036] Although the above embodiments specifically depict deployment of
ILE apparatus in trunk paths of OADM network, embodiments in which ILE
apparatus are deployed at any or all terminals of a branch network are
possible. Furthermore, a control system that controls power output of an
ILE used for fault recovery as described hereinabove need not be located
at the same terminal as the ILE under control. Since the control systems
may be embodied in software routines and/or programmed into hardware,
control systems may be conveniently added and deployed wherever
convenient for controlling optical output over desired communications
links of the OADM system.

[0037] Herein, novel and inventive apparatus and techniques for monitoring
and adjusting optical output power in communication paths of OADM systems
during fault period are disclosed. The present disclosure is not to be
limited in scope by the specific embodiments described herein. Indeed,
other various embodiments of and modifications to the present disclosure,
in addition to those described herein, will be apparent to those of
ordinary skill in the art from the foregoing description and accompanying
drawings.

[0038] Thus, such other embodiments and modifications are intended to fall
within the scope of the present disclosure. Further, although the present
disclosure has been described herein in the context of a particular
implementation in a particular environment for a particular purpose,
those of ordinary skill in the art will recognize that its usefulness is
not limited thereto and that the present disclosure may be beneficially
implemented in any number of environments for any number of purposes.
Accordingly, the claims set forth below should be construed in view of
the full breadth and spirit of the present disclosure as described
herein.